AI: Add null check for enemy player before accessing its properties

[seer-by-sentry]

Fixes CLIENT-V5. The issue was that: AIPlayer::guardSupplyCenter dereferences NULL pointer returned by getSkirmishEnemyPlayer without validation.

• Added a null check for the enemy player pointer before attempting to access its player index or structure bounds.
• This prevents potential crashes if the enemy player is not properly initialized or has been destroyed.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2708409

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